1. Field of the Invention
The present invention relates generally to optical fiber communication networks and, more particularly, to systems and methods for dynamically controlling gain in accordance with the collective behavior of the amplifier chains employed in the links of such networks.
2. Description of the Background Art
A dramatic increase in the information capacity of an optical fiber can be achieved by the simultaneous transmission of optical signals over the same fiber from many different light sources having properly spaced peak emission wavelengths. By operating each light source at a different peak wavelength, the integrity of the independent messages from each source is maintained for subsequent conversion to electric signals at the receiving end. This is the basis of wavelength division multiplexing (WDM).
Wavelength switched optical networks potentially offer high capacity networking at lower cost than current electronically switched networks. The optical amplifiers in the nodes and repeaters of such networks will each be traversed by multiple signal channels following diverse routes. In optical amplifiers such as rare-earth doped fiber amplifiers (e.g., erbium doped fiber amplifiers--EDFA's), amplified spontaneous emission (ASE) is the major source of noise. ASE originates from the spontaneous emission of incoherent light over the broad gain bandwidth of the amplifier and constitutes the random noise contribution of the amplifier. If the signal powers in the transmission fibers are too high, optical nonlinearities such as Stimulated Brillouin Scattering (SBS) can also occur and further degrade the signals by introducing noise. In the wavelength domain, gain saturation induced by a data channel operating at .lambda..sub.1 produces a level change in another data channel at wavelength .lambda..sub.2.
In optically amplified systems, the above-described noise sources present two limitations on the amplifier operating range. At low input signal levels the amplifier random noise contribution, ASE, causes bit errors (signal-spontaneous beat noise) while at large input signal levels, nonlinearities in the transmission fiber add noise and can also degrade performance. As such, fluctuations in the transmitted data stream--as may occur, for example, when one or more wavelength channels are added or dropped--can have a substantial effect on the reliability and quality of service in a multiwavelength network. Illustratively, the number of channels traversing an EDFA may change suddenly as a result of a network reconfiguration or a fault that interrupts some of the channels. Cross saturation in the affected optical amplifiers of a network will induce power transients in the surviving channels, the speed of which is proportional to the number of amplifiers in the network; for large networks, surviving channel power transients can be large and extremely fast. If their power levels exceed thresholds for optical nonlinearities or become too low to preserve adequate eye opening, the surviving channels traversing the optical amplifier will suffer error bursts.
The gain medium in a rare-earth doped optical fiber amplifier such, for example, as an EDFA has a comparatively long excited state lifetime or relaxation time, and for this reason is generally regarded as allowing for a larger saturation energy and, hence, as exhibiting virtually no saturation in response to high speed data pulses (1 ns). In fact, it has been reported that transient effects of gain saturation and recovery in an individual amplifier typically occur on a 100 .mu.sec-1 msec time scale. Desurvire et al., Erbium Doped Fiber Amplifiers, p. 412 (1994)!. The inventors herein, have, however, observed gain dynamics in a chain of EDFA's almost two orders of magnitude faster than this and, for large scale wavelength routed networks, gain dynamics three orders of magnitude may be predicted. These fast transients in chains of amplifiers may ultimately constrain the design or extent of multiwavelength optical networks. Accordingly, there is recognized a need for a technique by which the amplifiers employed in optical networks can be reliably controlled despite power level fluctuations in the respective wavelength channels or, in the case of time division multiplexed networks, individual time slots.